There is a desire in the industry for higher circuit density in microelectronic devices made using lithographic techniques. One method of achieving higher area density is to improve the resolution of circuit patterns in resist films. It is known in the art that increasing the numerical aperture (NA) of the lens system of the lithographic imaging tool increases the resolution at a given wavelength. However, increasing the NA results in a decrease in the depth of focus (DOF) of the imaging radiation, thereby requiring a reduction in the thickness of the imaging resist film. A decrease in the resist film thickness can lead to problems in subsequent processing steps (e.g., ion implantation and etching).
Bilayer thin film imaging (TFI) has been proposed as one approach to meet both the lithography and the etch resistance requirements for the fabrication of the next generation devices. In bilayer TFI, fine features are first delineated in a thin, usually silicon containing, top layer on a highly absorbing thick organic underlayer. The images thus formed are then transferred down to the underlayer through an antisotropic oxygen-containing reactive ion etching (O2 RIE), during which the silicon in the top layer is converted into nonvolatile silicon oxides, which subsequently functions as an etch mask. In this imaging scheme, the thin top layer improves lithographic resolution and process window (particularly for high numerical aperture (NA) exposure tools), while the thick underlayer affords substrate etch resistance. Bilayer TFI has the potential for enhanced resolution capability, improved process latitude, planarization over topography, and the ability to pattern high aspect ratio resist features for superior substrate etch resistance.
In thin film imaging schemes, patterns are transferred into an organic underlayer using a previously patterned Si-containing layer as the etch-mask. Although O2 is a very effective etching gas, in order to control the critical dimension it is necessary to add a passivating gas to the plasma such as SO2 or C2H4. The SO2—O2 etch chemistry has been determined to be the most effective for multiple bilayer resist programs.
The SO2—O2 etch is accompanied with the formation of a sulfur-containing passivating film on the feature sidewalls. The sulfite/sulfate species present in the passivating film tend to pick up moisture when the wafers are either rinsed in water or kept in the ambient. (Mori et al., “Pattern collapse in the top surface imaging process after dry development”, J. Vac. Sci. Technol. B 16(6), 1998, pp. 3744-3747). As a result, agglomerates may develop on the wafers surface when exposed to atmosphere. As the features get smaller and denser (e.g. 150 nm 1:1 line-and-space patterns and smaller), occasionally line collapse is also observed in high aspect ratio features (AR>8) after the SO2—O2 etch has been performed. Pattern collapse is attributed to the surface tension forces that arise when the sulfuric agglomerates on two neighboring lines swell hygroscopically with time and ultimately combine. Alternatively, the problem of line collapse may be avoided by performing the etch step without exposing the wafer to moisture.
The extent of pattern collapse increases with increasing aspect ratio and depends upon pattern density. It is also dependent on the thin film imaging scheme chosen (e.g. bilayer, CARL) and other factors such as wafer loading and type of structure being etched. Researchers have reported other methods to prevent line-collapse such as heating the wafer or treating the wafer to an O2 (Mori et al.) or O2—He plasma after etch. This also helps reduce or in this specific case eliminates the line-collapse problem. For contact levels, line collapse is not an issue.
If pattern collapse occurs the wafer must be reworked else the affected area will lower the overall process yield. This invention describes an in situ post-etch process that prevents the dense lines from collapsing.
Other objects and advantages will become apparent from the following disclosure.